In semiconductor fabrication, insulating, semiconducting, and conducting layers are deposited and patterned on a substrate to form device structures, such as transistors, capacitors, or resistors. These device structures are then interconnected to achieve a desired electrical function, creating an IC. The production and patterning of the various device layers are achieved using conventional fabrication techniques such as, for example, oxidation, implantation, deposition, epitaxial growth of silicon, lithography and etching. Such techniques are described in S. M. Sze, "VLSI Technology", 2nd ed., New York, McGraw-Hill, 1988, which is herein incorporated by reference for all purposes.
As demand for faster and denser ICs increases, the ability to control the spatial location of buried device layers becomes more important. For example, a buried strap used to couple a transistor to a capacitor to form a dynamic random access memory (DRAM) cell. A minimum buried strap thickness is required, depending on performance and design specifications.
Conventional techniques for forming buried straps includes at least three etch processes. The first etch recesses the polycrystalline (poly) fill and collar that lines the upper portion of the trench capacitor. The second etch further recesses the collar below the poly fill to define the bottom of the buried layer. Poly is deposited to fill the trench and a third etch is performed to recess the poly to define the top of the buried layer. Such techniques produce large variations in buried strap height, for example, of about +/-50 nm. To ensure that the buried strap is at least of the minimum buried strap thickness, the buried strap is at least minimum thickness+.vertline.variation.vertline..
The buried strap thickness depends on design requirements. Typically, the thickness is about 100 nm, which would mean that the minimum thickness is 150 nm with a variation in buried strap thickness from 100-200 nm. Such a large variation in buried strap thickness across the IC is undesirable as device performance is adversely impacted. Additionally, such a large variation requires a thicker buried strap, adding process difficulties. For example, its is desirable to have the shallow trench isolation (STI) depth as shallow as possible. However, a thicker buried strap results in the bottom of the buried strap being deeper beneath the substrate surface, requiring a deeper STI.
From the above discussion, an improved buried strap having smaller variations in thickness is desirable.